Many vehicles currently employ a front skirt in front of their tires to improve their aerodynamics. Tires increase drag because when air impacts the front of the rotating tires, the tires give rise to air vortices and introduce a second separation point in the air flow. This drag increases with velocity, and at high speeds the front tires contribute close to 30% of front cross-sectional drag. Benefits of reduced drag include better handling at high speeds, improved fuel efficiency and hence, reduced emissions.
The front skirt used on certain BMW® automobiles is a plastic, rectangular flap suspended from the bumper, extending approximately 90 mm down in front of the tire. The skirt redirects some of the air away from the tire, reducing the drag. Ideally, the skirt would extend far enough to keep all or most air from impacting the tire, but the skirt length is limited for two main reasons. First, by being closer to the ground, the skirt runs into obstacles such as speed bumps, curbs, potholes, and parking blocks. Second, the skirt detracts from the visual aesthetic of the design. These factors are most problematic at slow speeds, when vehicles are most likely to encounter parking blocks and speed bumps, and while stopped, when the skirt's detraction from vehicle aesthetics is most noticeable. The current design addresses these issues by keeping the skirt length short (thus not realizing the full potential to reduce drag) and making it out of flexible material that can bend when it hits an obstacle.